@Anonymous User
Sounds like you've reached your device limit. If you've watched on four different devices in the past then the device limit rules kick in, where you can now only add one new device a month, even if you delete several.
If you haven't already, have a word with Live Chat, explain what's happened and see if they can reset your devices for you. To get there go to the following help page
http://help.nowtv.com/article/Device-limit-reached
and click the green Contact Us button near the bottom left of it (not the How to Contact Us link right at the bottom as that takes you somewhere else) then choose the Live Chat option on the following page.
Best not to keep adding or removing devices too often otherwise you'll keep hitting this problem.
